A titanium-based alloy

ABSTRACT

A titanium-based alloy composition consisting, in weight percent, of: between 0.5 and 2.5% aluminium, between 0.5 and 1.5% vanadium, between 0.0 and 3.0% iron, between 0.0 and 1.0% chromium, between 0.0 and 3.0% nickel, between 1.0 and 4.0% molybdenum, between 0.0 and 1.0% silicon, between 0.0 and 0.2% boron, between 0.0 and 0.5% tin, between 0.0 and 0.5% zirconium, between 0.0 and 1.0% niobium, between 0.0 and 1.0% tantalum, between 0.0 and 0.5% calcium, between 0.0 and 0.5% carbon, between 0.0 and 0.5% manganese, the balance being titanium and incidental impurities, wherein one of iron and nickel is present in an amount of at least 2.0% and the other of iron and nickel is present in an amount of 1.0% or less.

The present invention relates to titanium-based alloy compositions designed for enhanced superplastic formability during sheet metal-forming processes which have lower cost than current alloys and improved cold formability.

Examples of typical compositions of α+β Ti-alloys which are superplastic and which are often used for the manufacture of superplastically-formed components are listed in Table 1.

TABLE 1 Nominal composition in wt. % of commercially used titanium alloys for superplastic forming applications. Alloy Al V Fe Mo Ti Ti—6Al—4V 6 4 — — Bal. Ti54M 5.1 3.9 0.5 0.7 Bal. ATI425 4 2.5 1.5 — Bal. SP700 4.5 3 2 2 Bal.

These alloys suffer from relatively poor superplastic and cold formability and a relatively narrow formability window—in terms of temperature, flow stress and strain-rate.

It is an aim of the invention to provide a α+β Ti-alloy which has equivalent or improved superplastic formability in comparison with the superplastic alloys listed in Table 1. A comparable or lower raw cost and improved or comparable machinability and cold formability are desired.

The present invention provides a titanium-based alloy composition consisting, in weight percent, of: between 0.5 and 2.5% aluminium, between 0.5 and 1.5% vanadium, between 0.0 and 3.0% iron, between 0.0 and 1.0% chromium, between 0.0 and 3.0% nickel, between 1.0 and 4.0% molybdenum, between 0.0 and 1.0% silicon, between 0.0 and 0.2% boron, between 0.0 and 0.5% tin, between 0.0 and 0.5% zirconium, between 0.0 and 1.0% niobium, between 0.0 and 1.0% tantalum, between 0.0 and 0.5% calcium, between 0.0 and 0.5% carbon, between 0.0 and 0.5% manganese, the balance being titanium and incidental impurities, wherein one of iron and nickel is present in an amount of at least 2.0% and the other of iron and nickel is present in an amount of 1.0% or less. This composition provides a good balance between cost, density, optimal mechanical performance at service temperature and low-stress and low-temperature superplastic formability and adequate microstructural stability.

In an embodiment the titanium-based alloy consists of at least 1.0% aluminium. This helps keep the density of the alloy low and helps increase the diffusivity merit index and thereby formability and the strength of the alloy.

In an embodiment the titanium-based alloy composition consists of at most 2.25% or less than 2.25% or 2.0% or less aluminium. This is advantageous as this improves the cold formability of the alloy and helps reducing further the temperature for superplastic forming.

In an embodiment the titanium-based alloy consists, in weight percent, of 0.5% or less iron. This is advantageous as it increases the stability of the alloy avoiding the formation of the w-phase—or so-called ‘beta fleck’.

In an embodiment the titanium-based alloy consists, in weight percent, of 0.5% or less chromium, preferably 0.4%, or less chromium. This is advantageous as it increases the stability of the alloy avoiding the formation of the w-phase—or so-called ‘beta fleck’.

In an embodiment the titanium-based alloy composition consists, in weight percent, of 0.1% or more Si, preferably 0.2% or more Si, more preferably 0.5% or more silicon. This is advantageous as it increases strength and creep resistance.

In an embodiment the titanium-based alloy composition consists, in weight percent, of 0.05% or more boron, preferably 0.1% or more boron. This is advantageous as it improves the ductility of the alloy.

In an embodiment the titanium-based alloy composition consists, in weight percent, of 0.25% or more nickel. This ensures a higher diffusivity merit index.

The term “consisting of” is used herein to indicate that 100% of the composition is being referred to and the presence of additional components is excluded so that percentages add up to 100%. Unless otherwise stated, all amounts are in weight percent.

The invention will be more fully described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a flow diagram illustrating the process by which the titanium-based alloy composition was determined;

FIG. 2 illustrates for each of aluminium, vanadium, chromium, iron, nickel and molybdenum how composition affects the temperature for optimal superplastic forming and density;

FIG. 3 illustrates for aluminium, vanadium, chromium, iron, nickel and molybdenum how composition affects the temperature for optimal superplastic forming and cost;

FIG. 4 illustrates for aluminium, vanadium, chromium, iron, nickel and molybdenum the effect of composition on optimal temperature for superplastic forming versus normalised diffusivity;

FIG. 5 illustrates the trade offs in alloy characteristics apparent from FIGS. 2-4 and the trade offs in the alloy of the present invention compared to the four commercially available alloys of Table 1;

FIGS. 6-8 illustrate the effect of varying composition on optimal superplastic forming temperature with Fe and Ni fixed at zero in FIG. 6, Cr and Ni fixed at zero in FIG. 7 and Cr and Fe fixed at zero in FIG. 8;

FIGS. 9-11 illustrate the variation in density as a function of composition for the alloy system with Ni and Fe fixed at zero in FIG. 9, Ni and Cr fixed at zero in FIG. 10 and Fe and Cr fixed at zero in FIG. 11;

FIGS. 12-14 illustrate the effect of composition on cost with Ni and Fe fixed at zero on FIG. 12, Ni and Cr fixed at zero in FIG. 13 and Fe and Cr fixed to zero in FIG. 14;

FIGS. 15-17 illustrate the effect of composition on diffusivity with Fe and Ni fixed at zero in FIG. 15, Cr and Ni fixed at zero in FIG. 16 and Cr and Fe fixed at zero in FIG. 17;

FIG. 18 illustrates the constraints used in the computer software to find the optimum alloy composition and the target properties; and

FIG. 19 illustrate graphs of temperature for optimal superplastic forming on the y axis versus cost, density, normalised diffusivity and stability showing the variation throughout the alloy design space and the location of the alloy of the present invention.

Traditionally, titanium-based alloys have been designed through empiricism. Thus their chemical compositions have been isolated using time consuming and expensive experimental development, involving small-scale processing of limited quantities of material and subsequent characterisation of their behaviour. The alloy composition adopted is then the one found to display the best, or most desirable, combination of properties. The large number of possible alloying elements indicates that these alloys are not entirely optimised and that alloys with more desirable properties are likely to exist.

In titanium alloys, generally additions of aluminium (Al) are added as an α-stabiliser to improve the mechanical strength. However, large additions of Al lead to cold workability deterioration—machining particularly. Also, excess content of Al leads to stability problems: loss of ductility and stress corrosion occurs due to reordering reaction of α₂ formation after long-time exposure at high-temperature.

General additions of vanadium (V) are added as a β-stabiliser to increase the mechanical strength without forming brittle intermetallic compounds. V makes a solid solution with the β phase.

Additions of nickel (Ni), cobalt (Co), iron (Fe) and chromium (Cr) are added as β-stabiliser elements to reduce the flow stress during superplastic forming and to maximise the strain-rate sensitivity. These elements have a diffusivity higher than that of V, hence tend to increase the diffusivity of Ti-6Al-4V.

Small additions of silicon (Si) are added to increase the strength and creep resistance. At high-temperatures, Si dissolves in the α phase and precipitates as silicides that pin mobile dislocations from climb and glide. Silicon was not part of the calculations described hereinafter, but experience shows that additions of silicon of up to 1.0%, but preferably 0.1% or more, more preferably 0.2% or more and most preferably at least 0.5% are beneficial for increased strength and ductility and so are included in the inventive alloy.

Small additions of boron (B) are added to improve the ductility due to enhancement of the prior-β grain-boundary cohesion by boron segregation at the grain boundaries. Boron was not part of the calculations described hereinafter, but experience shows that additives of up to 0.2%, but preferably limited to 0.1% are beneficial and so are included in the inventive alloy. Small amounts of boron of 0.05% or more or 0.1% or more are beneficial for improved ductility. The present inventors have not modelled the effect of low levels of other commonly used alloying elements such as tin, zirconium, niobium, tantalum, calcium, carbon and manganese.

Neutral phase elements such as zirconium and tin may be added in quantities of 0.5% or less—these won't change the α-β phase proportion. The density of Zr and Sn is close to that of Mo therefore the following limitation may be introduced in order to keep the density below the imposed constrain (Zr wt. %+Sn wt. %+Mo wt. %≤4.0 wt. %).

Niobium and tantalum have similar effects in titanium alloys and each may be added in quantities of 1% or below as a β-stabiliser. Nb and Ta do not affect the stability of the alloy and have a density and price comparable to that of Mo—Nb and/or Ta may act as a substitute to Mo. Therefore, preferably the amount of Nb and Ta are limited in the following manner: Nb wt. %+Ta wt. %+Mo wt. %≤4.0 wt. %. Preferably Nb wt %+Ta wt. %<1.0 wt. %.

Manganese is a β-eutectoid stabiliser therefore further additions of Mn will reduce the SPF temperature but it will promote formation of unwanted phases similarly to Cr, Ni and Fe. Due to a very similar density and cost to Cr, one may substitute an amount of Cr (up to 0.5 wt. %) by Mn. So preferably Cr wt. %+Mn wt. %≤1.0 wt. %, more preferably ≤0.5 wt. %.

Calcium and carbon may be present at levels of up to 0.5% each and are not expected greatly to change the character of the alloy at this level.

A modelling-based approach used for the isolation of new grades of titanium based alloys is described here, termed the “Alloys-By-Design” (ABD) method. This approach utilises a framework of computational materials models to estimate design relevant properties across a very broad compositional space. In principle, this alloy design tool allows the so called inverse problem to be solved; identifying optimum alloy compositions that best satisfy a specified set of design constraints.

The first step in the design process is the definition of an elemental list along with the associated upper and lower compositional limits. The compositional limits for each of the elemental additions considered in this invention—referred to as the “alloy design space”—are detailed in Table 2.

TABLE 2 Alloys design space in wt. % searched using the ‘Alloy-by-Design’ method. Al V Fe Cr Ni Mo Si B Min. 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Max. 8.0 8.0 4.0 4.0 4.0 6.0 0.5 0.1

The design process comprises discretising the alloy design space into different compositions covering the complete alloy design space (for example with amounts of each given element varying by 0.01 or 0.001 between different compositions with the amounts of the other added elements remaining constant. Thus, a huge member of specific alloy compositions within the alloy design space are determined. For each alloy composition a property is calculated.

The second step relies upon thermodynamic calculations used to calculate the phase diagram and thermodynamic properties for each of the specific alloy compositions. Often this is referred to as the CALPHAD method (CALculate PHAse Diagram). These calculations are conducted for those temperatures where an optimal phase architecture of the new alloy is found: temperatures in excess of 40% the melting point of the alloy and where the ratio of α-to-β phase is approximately 0.6.

A third stage involves isolating specific alloy compositions which have the desired microstructural architecture. In the case of titanium alloys which require maximum formability via the exploitation of superplasticity, the enhanced regime is found when the volume fraction of the β-phase lies between 30%-50% at temperatures where the thermally-activated deformation is active: i.e. above 0.4 of the melting temperature of the alloy (0.4 T/Tm). This can be measured experimentally by differential scanning calorimetry or by quenching from the temperature so that β-phase transforms to martensite not α-phase, observable by metallographic examination.

Rejection of a specific alloy composition on the basis of unsuitable microstructural architecture is also made from estimates of susceptibility to form unstable precipitates. The present calculations predict the formation of the α₂ precipitates using CALPHAD modelling. Moreover, the susceptibility of the alloy to form the deleterious segregation phase called ‘beta fleck’ is calculated in terms of molybdenum equivalent weight percent.

The model rejects all specific alloy compositions not meeting these design criteria and only maintains those specific alloy compositions in the design space which are calculated to result in a volume fraction of β of between 30 and 50% at temperatures of creep (>0.4 T/Tm) which form experience are expected to have a low tendency to form the unstable segregation phase ‘beta fleck’.

In the fourth stage, merit indices are estimated for the remaining isolated alloy compositions in the dataset. A merit index is a value calculated according to a formula (described below) which is indicative of a desired property of the alloy. Examples of the merit indices include: β-diffusivity merit index (which describes an alloy's superplastic formability based solely on mean composition), superplastic forming temperature merit index, density, cost and diffusivity.

At the end of the process only those alloy compositions which have passed the thermodynamic requirements and all of the merit indices remain and this is the optimized alloy composition.

The first merit index is the temperature at which superplasticity is optimal; or the temperature where the microstructure is composed by 40% β-phase and 60% α-phase. Equilibrium thermodynamic calculations were carried out in order to determine the temperature of 40% β-phase for each single composition within the proposed design space. The lower this superplastic forming (SPF) temperature, the better as less energy is needed for forming and less damage (e.g. due to oxidation occurs at lower forming temperatures.

The second merit index is the diffusivity of the β-phase; this is strongly linked to the stress necessary to activate superplastic deformation. A faster diffusivity of β is translated directly into a decrease of the flow stress of superplastic titanium. This is accomplished by addition of small percentages of elements with tracer diffusivity higher than that of titanium on β-Ti—e.g. Fe, Ni and Cr—since the diffusivity is controlled by the faster diffusing species, consistent with

D _(eff)=Σ_(i) x _(i) D _(i)  (1)

where x_(i) is the concentration of element i and D_(i) is its tracer diffusivity as shown in Table 3. The forming resistance merit index is related directly to this diffusivity, for clarity, the forming resistance merit index is written as a function of the diffusivity of V on β titanium—see Table 3.

TABLE 3 diffusivity (D) of beta stabilising elements at 870 C. Diffusivity (D) of β-stabilising elements at 870° C. D of element/ Element D (cm²s) D of V Ni  220 × 10⁻¹⁰ 63.7 Co  190 × 10⁻¹⁰ 55.3 Fe   78 × 10⁻¹⁰ 22.6 Cr   11 × 10⁻¹⁰ 3.22 β-Ti 3.36 × 10⁻¹⁰ 1.0 V  2.4 × 10⁻¹⁰ 0.69 Nb  1.7 × 10⁻¹⁰ 0.49 Al 1.44 × 10⁻¹⁰ 0.43 Mo  0.6 × 10⁻¹⁰ 0.173 W  0.2 × 10⁻¹⁰ 0.063

The third merit index is density. The density, p, was calculated using a simple rule of mixtures and a correctional factor of 5% as has been shown practically to give a more accurate prediction, where, ρ_(i) is the density for a given element and x_(i) is the atomic fraction of the alloy element.

ρ=1.05[Σ_(i) x _(i)ρ_(i)]  (2)

The fourth merit index is cost. In order to estimate the cost of each alloy a simple rule of mixtures was applied, where the weight fraction of the alloy element, x_(i), was multiplied by the current (2015) raw material cost for the alloying element, c_(i).

Cost=Σ_(i) x _(i) c _(i)  (3)

The estimates assume that processing costs are identical for all alloys, i.e. that the product yield is not affected by composition.

The fifth merit index is the β-phase stability. In order to avoid the formation of the ω-phase—or so-called ‘beta fleck’—one must keep the amount of non-solid solutioning β stabilisers below a maximum. This sum is defined in terms the addition of the wt. % of Ni, Fe and Cr as

Stability=Fe wt. %+Ni wt. %+Cr wt. %.  (4)

If the stability merit index exceeds a value of 3 to 3.5, the alloy is likely to be susceptible to form the w segregation phase upon melting and solidification of the alloy—this would translate into a loss of ductility under service conditions. However, this index is related directly to the forming resistance of the alloy: the lower the resistance, the higher the stability merit index will become. For optimal formability and microstructural stability, this fifth merit index is assumed optimal between a value of 2 to 3.

The sixth merit index is the aluminium content. This is related directly to the machinability, strength and stability of the alloy. High values (>7 wt. %) of aluminium will cause a brittle behaviour in the alpha phase. Medium aluminium content (3-6 wt. %) provides good strength but the machinability is difficult. No aluminium content facilitates greatly the cold machinability but the strength is decreased substantially. This led to a maximum aluminium content of 2.5%. Reducing the aluminium concentration to 2.25% or less, less than 2.25% or to 2.0% or less improves cold formability and helps in further reducing the temperature for superplastic forming.

The ABD method described above was used to isolate the inventive alloy composition. The design intent for this alloy was to isolate a composition of a new titanium alloy which exhibits a combination of superplastic formability, strength and ductility which is comparable or better than equivalent grades of alloy. The density, cost and processing of the alloy have also been considered in the design of the new alloy. A thermoformed product made of the alloy composition preferably has an equiaxed alpha-beta microstructure with a grain size below 10 microns—more preferably below 6 or 7 microns.

The material properties—determined using the ABD method—for the commercially available superplastic titanium alloys are listed in Table 4. The design of the new alloy was considered in relation to the predicted properties listed for these alloys. The method was used to propose a novel alloy composition which targets different properties. The calculated material properties for the ABD-SPTi alloys with nominal compositions according to Table 5 and in accordance with the present invention are also given.

TABLE 4 Calculated phase fractions and merit indices made with the “Alloys-by- Design” software. Results for four commonly used SPF Ti alloys as listed in Table 1 and the compositions of the new alloys ABD-SPTi listed in Table 5. Cold β phase Diff, Mert formability SPF temp. Wt. % Mo Index Density Wt. % Al Alloy (° C.) equivalent (D_(eff)/D_(8Ti)) (g/cm³) Cost ($/lb) content Ti—6Al—4V 876 0 0.81 4.29 5667 6 Ti54M 815 1.5 1.18 4.34 5692 5 ATI425 802.8 0.5 1.65 4.36 5309 4 SP700 725 2 1.96 4.39 5500 4.5 ABD-SPTi 672 3.0 4.61 4.51 5277 1.5 ABD-SPTi LC 715 3.0 2.55 4.46 4788 1 ABD-SPTi LD 723 3.0 2.52 4.41 4955 2.5 ABD-SPTi LT 631 3.0 2.49 4.52 5243 1.5 ABD-SPTi HD 713 3.0 5.37 4.47 5268 2

TABLE 5 Compositions of the titanium alloys derived using the proposed methodology. Comments Al V Fe Cr Ni Mo Si B ABD-SPTi Nominal 1.5 1.0 0.5 0.0 2.5 3.0 0.5 0.1 ABD-SPTi LC Lowest cost 1.0 0.5 3.0 0.0 0.0 1.0 0.5 0.1 ABD-SPTi LD Lowest density 2.5 1.0 3.0 0.0 0.0 1.0 0.5 0.1 ABD-SPTi LT Lowest temp. 1.5 1.5 3.0 0.0 0.0 3.0 0.5 0.1 ABD-SPTi HD Highest diff. 2.0 1.0 0.0 0.0 3.0 2.0 0.5 0.1

The method described above was used to isolate the inventive alloy composition. The design intent for this alloy was to achieve good cold formability, superplastic forming at relatively low temperatures and with low stress. The cost of the alloy was to be comparable to or lower than those currently commercially available. Those properties were given more importance than the density and stability merit index.

The material properties, determined using the above described method, for the commercially used alloys of Table 1 are listed in Table 4 along with those of an alloy falling within the present invention. This shows a greatly reduced superplastic forming temperature along with a greatly increased diffusivity merit index (meaning a lower stress is required for superplastic forming) along with a low amount of aluminium allowing good cold formability.

The cost of the alloy is also lower than the majority of the commercially available alloys and at least comparable to the cheapest commercially available alloys. The density of the alloy is a little greater than those of the commercial alloys and the stability merit index is also higher.

Because of the lower amounts of aluminium and vanadium than the commercially available alloys, the alloy of the present invention can also be expected to be less strong than the commercially available alloys. The lower stability means that the alloy is also more prone to martensite formation.

FIG. 1 is a flow diagram of the process for designing the alloy of the present invention. As a first step a design space is defined. The design space is shown in Table 2. The design space is then discretised into many different individual alloy compositions and for each of those individual alloy compositions thermodynamic calculations as described above are performed.

Based on the thermodynamic calculations and calculated merit indices, the effect of each alloying element on the import design parameters may be plotted. FIGS. 2-4 are such plots.

FIG. 2 shows the effect of alloying components on the alloy density along the x axis and the superplastic forming temperature along the y axis. What this shows is that generally as alloys increase in density the superplastic forming temperature decreases.

FIG. 3 plots the effect of alloy composition against cost along the x axis and superplastic forming temperature along the y axis. This shows that there is not such a strong correlation between cost and superplastic forming temperature as between density and superplastic forming temperature.

FIG. 4 illustrates along the x axis the normalised diffusivity versus superplastic forming temperature along the y axis for different compositions. What is striking about these results is the strong influence of nickel on the normalised diffusivity.

The next stage of the design process was to determine the properties required of the alloy. This is illustrated with the help of FIG. 5. In FIG. 5 the influences from FIGS. 2-4 are plotted one against each other around the outside of the triangle. Plotted inside the triangle is the position of the four commercial alloys of Table 1 and their relative performance in terms of cost, formability and density. Also plotted is the balance of properties achieved by the present invention, namely relatively low cost compared to the commercially available alloys, a similar density to the commercially available alloys and better formability compared to the commercially available alloys.

FIGS. 6-17 are plots showing variations in certain merit indices with variation in aluminium content along the x axis and vanadium content along the y axis for different fixed amounts of other elements. Elements not mentioned are present at zero percent.

On the basis of the data in FIGS. 6-17 and on the calculated merit indices in Table 4 for existing commercial alloys, desired merit indices for the inventive alloy were devised. These are shown in FIG. 18 pictorially.

The strongest variation in a merit indices with composition is the variation in normalised diffusivity with nickel content (FIG. 4). In order to achieve as high diffusivity as possible, as high an amount of nickel as possible whilst meeting the remainder of the merit indices, is required. The strong effect of diffusivity of nickel can be seen in FIG. 4 and is also illustrated in FIG. 17. Also as can be seen from FIG. 8, increasing nickel content also has the beneficial effect of reducing superplastic forming temperature (a decrease in superplastic forming temperature in the graphs going from the left to the right of FIG. 8). Therefore a preferably minimum level of nickel is set at 0.25% or more. In order to meet the requirement of a stability merit index of 3.0 or less (see equation 4) whilst dramatically increasing diffusivity merit index, the amount of nickel is limited to 3.0%. The nickel content may be replaced by iron. This would advantageously further decrease the cost and the forming temperature (see FIGS. 13 and 14 and FIGS. 7 and 8) at expense of reducing the normalised diffusivity (see FIGS. 16 and 17), but still maintain the diffusivity merit index above 2.

One of nickel and iron (but not both) is present at 2.0% or more, to ensure that the diffusivity merit index of at least 2 is met. The other of nickel and iron is present at up to 1.0% to preserve microstructural stability. A desired minimum level of nickel of 2.0% is chosen in order to meet a minimum of normalised diffusivity of greater than 3.0 (see the central column of graphs in FIG. 17).

A nickel content of up to 3.0% means that the lower levels of iron and chromium which are also present in equation 4 is set at 0.0% to increase stability as much as possible. At the lowest level of nickel of 2.0%, one of iron and chromium may be present at up to 1.0%, thereby setting the upper limit for the amount of iron and chromium. If both iron and chromium are present, they may be present at up to 0.5%, thereby setting the preferred upper limit for iron and chromium. In the case of, an iron content of up to 3.0%, the lower levels of nickel and chromium which are also present in equation 4, is set at 0.0% to increase stability as much as possible. At the lowest level of iron of 2.0%, one of iron and chromium may be present at up to 1.0%, thereby setting the upper limit for the amount of iron and chromium. If both nickel and chromium are present, they may be present at up to 0.5%, thereby setting the preferred upper limit for nickel and chromium. In any case, a preferred upper level of chromium is 0.5% or less, preferably 0.4% or less to increase stability.

As can be seen from FIGS. 10 and 11, high levels of iron and nickel are generally detrimental to density of the alloy. As can be seen from FIGS. 9-11, aluminium is the element with the strongest influence on density. Therefore, the alloy contains as much aluminium as possible to compensate for the high levels of iron or nickel.

However, aluminium is detrimental for most other properties which are intended to be optimised in the inventive alloy. For a maximum level of nickel of 3.0%, FIG. 11 shows that a minimum amount of aluminium of 0.5%, even with a maximum amount of vanadium of 0.5% and a maximum amount of molybdenum of 4.0% (explained below) substantially limits the density of the alloy to below 4.6 g/cm³. If nickel is substituted by iron, density decreases. Thus, the minimum level of aluminium is set to 0.5%, preferably 1.0% as this further reduces density.

Increasing levels of molybdenum beyond 4.0% deleteriously increases the density and cost of the alloy so the maximum amount of molybdenum is set to 4.0%. However, increasing levels of molybdenum help in reducing the superplastic forming temperature (see the right hand column of FIG. 8) without reducing the stability (molybdenum does not appear in equation 4), whereas the other elements which are useful for reducing the superplastic forming temperature are limited for other reasons. For example, iron and chromium are limited by equation 4 and vanadium is limited by cost (see FIGS. 12-14). Therefore a minimum amount of molybdenum is set at 1.0% in order to reduce the superplastic forming temperature substantially below 725° C. (with maximum amounts of aluminium of 2.5%, minimum amounts of vanadium of 0.5% (described below) and minimum amounts of nickel of 2.0% (described above)). Desirably Mo is present in an amount of at least 1.5% or even 2.5% as increasing levels reduce the superplastic forming temperature. Most preferably Mo is present at least at 2.75% in order further to reduce the superplastic forming temperature while keeping an stable microstructure. Desirably Mo is present in an amount below 3.25% in order to keep the cost, diffusivity and density of the alloy well within the design constrains.

Vanadium is beneficial in increasing the superplastic forming temperature (see FIGS. 6-8) whilst is substantially neutral in terms of its effect on density. Vanadium is deleterious for the normalised diffusivity (see FIGS. 4 and 15-19). However, the effect of vanadium on cost of the alloy is the largest factor in limiting the amount of vanadium to 1.5%. By limiting the amount of vanadium to 1.5% the cost of the alloy can be kept to 5300 or below (see FIGS. 12-14).

In order to benefit from the effect of vanadium in improving the superplastic forming temperature without effecting stability (equation 4), a minimum level of vanadium of at least 0.5% is chosen. This means that even at the lowest levels of molybdenum and nickel and highest levels of aluminium, a superplastic deformation temperature of substantially at most 725° C. is achievable. Desirably V is present in an amount of at least 1.0% in order to increase the strength of the alloy and to reduce further the superplastic temperature.

Aluminium is limited due to its deleterious effect on cold formability, superplastic formation temperature and diffusivity. Limiting aluminium content to 2.5% or less enables the merit indices of FIG. 18 to be achieved at the extremes of the alloy range while also achieving a cold formability index lower than any of the commercially available alloys of Table 1. Desirably Al is present in an amount of at least 1.0% in order provide alloy strength and to help reducing the total density of the alloy.

On the basis of these considerations the composition of the alloy is determined in accordance with FIG. 19. The table shows that the amount of aluminium lying between 0.5 and 2.5% is much lower than that of the previous alloys. This manifests itself in particular in the good cold formability of the inventive alloy as illustrated by Table 4.

The amount of vanadium being between 0.5 and 1.5% is also much lower than that of the commercial alloys of Table 1. This helps maintain the cost of the alloy low.

The amount of iron and chromium is also relatively limited when the amount of nickel lies between 2.0 and 3.0% (which is relatively high). The high amount of nickel is substantially responsible for the very high diffusivity merit index and low SPF temperature. Desirably Ni and Fe are present in amounts below 2.5% in order provide good alloy stability reducing the risk of brittle behaviour. Ni is preferable to Fe due to its higher diffusivity but if cost and density are a primary concern one may substitute Ni for Fe in amounts between 2.0 to 2.5 wt. % so that low SPF temperatures are achieved but the alloy still offers a good microstructural stability.

The allowable amount of molybdenum and the preferred higher minimum levels are also high compared to the commercially available alloys of Table 1. A combination of high amounts of nickel and molybdenum are largely responsible for the low superplastic forming temperature of the alloy of the present invention.

The amounts of silicon and boron are not arrived at from thermodynamic calculations but instead are added from knowledge that they will increase the strength and creep resistance and enhance the ductility of the alloy. 

1. A titanium-based alloy composition consisting, in weight percent, of: between 0.5 and 2.5% aluminium, between 0.5 and 1.5% vanadium, between 0.0 and 3.0% iron, between 0.0 and 1.0% chromium, between 0.0 and 3.0% nickel, between 1.0 and 4.0% molybdenum, between 0.0 and 1.0% silicon, between 0.0 and 0.2% boron, between 0.0 and 0.5% tin, between 0.0 and 0.5% zirconium, between 0.0 and 1.0% niobium, between 0.0 and 1.0% tantalum, between 0.0 and 0.5% calcium, between 0.0 and 0.5% carbon, between 0.0 and 0.5% manganese, the balance being titanium and incidental impurities, wherein one of iron and nickel is present in an amount of at least 2.0% and the other of iron and nickel is present in an amount of 1.0% or less.
 2. The titanium-based alloy composition of claim 1, consisting of at least 1.0% aluminium.
 3. The titanium-based alloy composition of claim 1, consisting of at most 2.25% or less than 2.25% or 2.0% or less aluminium.
 4. The titanium-based alloy composition of claim 1, consisting of 0.5% or less iron.
 5. The titanium-based alloy composition of claim 1, consisting of 0.5% or less chromium, preferably 0.4% or less chromium.
 6. The titanium-based alloy composition of claim 1, consisting of 0.1% or more Si, preferably 0.2% or more Si, more preferably 0.5% or more Si.
 7. The titanium-based alloy composition of claim 1, consisting of 0.05% or more boron, preferably 0.1% or more boron.
 8. The titanium-based alloy composition of claim 1, consisting of 1.0% or more vanadium.
 9. The titanium-based alloy composition of claim 1, wherein Fe wt. %+Ni wt. %≤2.5%.
 10. The titanium-based alloy composition of claim 1, consisting of 1.5% or more Molybdenum, preferably 2.5% or more molybdenum, more preferably 2.75% or more molybdenum.
 11. The titanium-based alloy composition of claim 1, consisting of 3.25% or less Molybdenum.
 12. The titanium-based alloy composition of claim 1, wherein the microstructure of the alloy composition is 30-50 vol % β-phase at SPF temperatures with the remainder being α-phase.
 13. The titanium-based alloy composition of claim 1, wherein Fe wt. %+Ni wt. %+Cr wt. %≤3.5, preferably ≤3.0.
 14. The titanium-based alloy composition of claim 1, wherein Nb wt. %+Mo wt. %≤4.0.
 15. The titanium-based alloy composition of claim 1, wherein Zr wt. %+Sn wt. %+Mo wt. %≤4.0.
 16. The titanium-based alloy composition of claim 1, wherein Cr wt. %+Mn wt. %≤0.5.
 17. The titanium-based alloy composition of claim 1, consisting of 0.25% or more nickel.
 18. A thermoformed product made of the alloy composition of claim
 1. 